Alemtuzumab is a humanized monoclonal antibody that binds to CD52, a surface molecule expressed by many types of immune cells, including T lymphocytes, B lymphocytes, monocytes, macrophages, and eosinophils.1-3 After intravenous infusion, the therapeutic antibody binds to CD52-expressing cells in blood and to a lesser extent in lymphoid organs. Antibody binding is followed by profound depletion of CD52-positive immune cells brought about by complement- and cell-mediated cytotoxic mechanisms. Of all the immunotherapies currently available for multiple sclerosis, alemtuzumab has the longest lasting effects on the immune system.